Device-to-device (D2D) communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems or technologies operate in unlicensed spectrum.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment.
It is suggested that such a device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.
Devices that want/wish to communicate, or even just discover each other, typically need to transmit various forms of control signaling. One example of such control signaling is the discovery signal; which may include a full message e.g. a synchronization message or a beacon; which at least carries some form of identity and is transmitted by a device that wants/wishes to be discoverable by other devices. Other devices may scan for the discovery signals. Once they have detected the discovery signal, they may take the appropriate action, for example to try to initiate a connection setup with the device transmitting the discovery message.
Multiple discovery signals from different user equipments (UEs), being an example of a device, are multiplexed on the same radio resources in a combination of Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM) and/or Code Division Multiplexing (CDM). Even though details are not agreed yet at the standardization meeting e.g. 3GPP or IEEE, it is likely that discovery signals be multiplexed on specific subframes occurring at known (or signaled) positions in a radio frame. Similarly to the discovery signals, it is envisioned that UEs transmit channels for data and/or control information.
Resources for transmission of data and/or control channels (including discovery) may be assigned by a controlling node or be defined according to pre-configured patterns.
In general, each channel from each UE occupies a subset of the available time/frequency resources and possibly code resources in the system.
Since interference at the receiver in a D2D system may happen in a stochastic and partially uncontrollable/unpredictable fashion, it is understood [1] that frequency and/or time diversity is beneficial in the resource patterns used for each physical channel.
One approach is to avoid transmitting the (data or control) packets with the same periodicity by all transmitters (devices) in the system or network and they should span different portions of the spectrum. Possibly, code patterns may be exploited, too.
An example of time and frequency diversity achieving transmission patterns are shown in FIG. 1 as described in [1], see FIG. 10 in [1]. [1] is a 3GPP document disclosing simulation results and proposals and details relating to D2D broadcast resource allocation and interference managements algorithms. More details on the time and frequency diversity schemes may be found in [1] section 3.2.5, wherein it is described system level impact of frequency and time diversity on D2D Voice Over IP (VOIP) packet (re)transmissions.
Another known consideration is that unpredictable interference may occasionally prevent reception of specific packets for a certain channel from a certain transmitter device. Since all packets need to be correctly decoded in order to correctly convey information at the receiver device, retransmissions of the same packet are useful to improve system reliability.
For certain channels, feedback-based ACK/NACK mechanism may not be available (e.g., control channels, broadcast communication channels, discovery channels, etc.). In these cases, a possible approach is to provide blind retransmissions, i.e. transmit the same payload or packet multiple times on different resources, possibly with different encoding parameters (redundancy versions). Under certain conditions the receiver device might be able to reconstruct the correct information based on reception of at least one of the retransmissions of the same packet.
Devices or UEs need to be able to detect and/or decode physical channels that they are potentially interested in. Especially the decoding process takes a significant processing time, comparable to the length of some subframes. The longer the processing time, the higher the energy/power consumption at the receiver device and this leads to shortening battery life at the receiver device. Current implementations are already highly optimized and it is not likely that UEs will implement even faster decoding algorithms for the purpose of D2D channels detection.
The currently discussed randomization algorithms might result in more restrictive latency requirements on the decoders, since the random retransmissions might occur at any time due to time randomization.
It should be mentioned that among the most computationally demanding functions in the receiver device equalizer block and the decoder block as explained above, which are sometimes implemented by taking advantage of highly optimized and hardly upgradable hardware accelerators. These blocks, among others, contribute to the detection latency in the UEs. Currently, in Long Term Evolution (LTE) UEs have a time budget of about 4 ms for detection of an incoming DownLink (DL) data channel, due to that the feedback channel(s) carrying detection acknowledgements are by design spaced in time from the data reception subframes.
Further, due to the sometimes unpredictable and highly dynamic interference patterns of direct channels and due to constraints in the UEs (e.g., reception may be impossible during transmission) there exist significant chances that a receiver device or UE is not able to detect a certain Layer 1 (L1) data packet (i.e., a subframe carrying a direct channel) even though the transmitter device or UE is in proximity of the receiver.